1. Field of the Invention
The present invention generally relates to a semiconductor light-emitting device, and in particular, to a semiconductor light-emitting device such as a semiconductor laser for optical communication requiring high temperature and high speed operation with a sloped or graded multi quantum barrier structure to suppress overflow of carriers from an active layer to improve temperature characteristics.
2. Description of the Prior Art
Recently, in order to effectively deal with high speed data communication, it has been necessary to utilize optical communication networks using optical fibers. The optical communication network must be extended to a subscriber""s network as well as to a trunk line network.
A semiconductor light-emitting device such as a semiconductor laser is used as a key device for the optical communication network. As a semiconductor laser therefor, it is required to use a semiconductor light-emitting device which can operate at a high speed and high temperature without requiring an additional device, such as a cooling device or the like, in views of handling a large amount of data, low cost, and high reliability.
In a high speed modulation operation, heat generation of a semiconductor light-emitting device itself causes a rise in temperature. Electrons (i.e., carriers) have thermal energy because of such increase in temperature. For this reason, the electrons easily overflow from an active layer serving as a light emitting area of the semiconductor light-emitting device over an energy barrier, which results in a quantum efficiency decrease. The quantum efficiency herein refers to a ratio of radiative energy converted from an energy input and energy absorbed in a light-emitting device in a light emission process.
In order to emit light efficiently in a semiconductor laser, it is important to optimize confinement of carriers and light within the semiconductor. Therefore, a heterostructure is frequently used in which semiconductor active layer is sandwiched between paired semiconductor cladding layers, each having larger band gap energy and smaller refractive index than the active layer. In this heterostructuer, the carrier confinement and light confinement are efficiently performed.
A compound semiconductor is generally used as a semiconductor material for obtaining direct transition of radiative energy. For example, GaN, GaAs, GaAlAs, InGaAsP, InGaAs, GaSb and AlGaSb are selectively used in accordance with a light emission wavelength. A structure of an active layer is usually constructed as a quantum well structure.
Hereinafter, a basic structure and an operation of a conventional semiconductor laser will be described with reference to FIGS. 8 and 9. FIG. 8 is a schematic perspective view showing an example of the basic structure of the conventional semiconductor laser. FIG. 9 is a model view for explaining an energy band gap of the basic semiconductor laser.
As shown in FIG. 8, in a basic structure of a semiconductor laser 80, an active layer 81 for generating light is sandwiched between cladding layers 82a and 82b for confining the light. The active layer 81 generally has a multi quantum well structure (which hereinafter will be simply referred to as xe2x80x9cMQWxe2x80x9d). The MQW is formed in a manner such that, a barrier layer 83 made of a material having a large energy band gap and a thin quantum well layer 84 (also referred to as xe2x80x9cwell layerxe2x80x9d) made of a material having a small energy band gap are alternately sandwiched to form a quantum well structure, and a plurality of pairs of the quantum well structures are held between the cladding layers.
The quantum well structure is an artificial structure which confines carriers in one direction where the carriers serve as free electrons. The semiconductor laser described here is explained by taking a quantum well laser as an example in which the electrons and holes are confined within a narrow quantum well layer to efficiently generate laser oscillation.
Referring to FIG. 8, each of the barrier layers 83 has a thickness of, e.g., about 10 to 20 nm, and each of the quantum well layers 84 has a thickness of, e.g., about 5 to 10 nm. Carriers such as electrons e and holes h are confined within an area (quantum well layer, i.e., well layer) having a small energy band gap, and the light is confined within an area having a large refractive index. The energy band gap has a correlation with the refractive index. A material with a smaller energy band gap has a larger refractive index. Accordingly, in a typical model example shown in FIG. 9, the carriers (e, h) and the light are both distributed mainly around the center area of the active layer.
Next, a separated confinement heterostructure (which hereinafter is simply referred to as xe2x80x9cSCHxe2x80x9d) will be described with reference to FIG. 10. In FIG. 8, if the active layer 81 having a MQW structure is made thin, quantum efficiency and high speed operational characteristic are improved, but a cut-off of a guided mode occurs to be problematic. In order to solve such a problem, variation is made in refractive index distributions of an n-cladding layer and a p-cladding layer provided outside the active layer. Thus, light generated at the active layer is leaked, so that the cut-off is prevented by the SCH in which the area for confining carriers (electrons e and holes h) is separated from the area for confining the light.
Specifically, it is noted that a structure having a continuously varying refractive index distribution of the cladding layer is referred to as GRIN (Graded Refractive Index), and a structure having a stepwise varying refractive index distribution is referred to as STEP. FIG. 10 is a model view showing an energy band gap of a semiconductor laser having cladding layers with STEP structure.
In a manufacturing procedure for a semiconductor laser, zinc (Zn), magnesium (Mg), beryllium (Be), carbon (C) and the like are used as materials for p-type doping.
In accordance with the above-described STEP heterostructure shown in FIG. 10, although the cut-off of the guided mode is improved, the energy band gap is formed in correlation with the refractive index of the cladding layer which is varied stepwise. For this reason, there is a problem that the carriers easily overflow from the active layer.
Moreover, zinc (Zn) serving as a p-type dopant has large thermal diffusion. The active layer becomes p-type by the zinc (Zn) diffusing to the active layer which should be originally a non-doped area. Thus, zinc (Zn) cannot be used for doping in the vicinity of the active layer. Further, there arise a problem in that other dopants with less thermal diffusion produce a low density of carriers, and are difficult to be introduce in a manufacturing process of the semiconductor material.
The present inventors devised a structure of a multi quantum barrier (hereinafter simply referred to as xe2x80x9cMQBxe2x80x9d) which is provided for the cladding layer in order to prevent carriers from overflowing from the active layer. FIG. 11 shows a model of the energy band gap of the semiconductor laser which uses the MQB structure for the cladding layers. In the structure shown in FIG. 11, periodic barriers of MQB are provided for the n-cladding layer 82a and the p-cladding layer 82b and the active layer having a MQW structure is sandwiched between the n-cladding layer 82a and the p-cladding layer 82b, so that an energy barrier of each cladding layer is made high because of quantum effects. Thus, it is possible to effectively prevent the carriers from overflowing from the active layer.
In accordance with the semiconductor laser adopting the MQB structure for such cladding layers as shown in FIG. 11, however, there arises a cut-off problem of a guided mode.
The present invention has been made in order to solve the above-described problems, and an object of the present invention is to suppress overflowing of carriers from an active layer serving as a multi quantum well (MQW) layer by utilizing a MQB for STEP structure of cladding layer, to prevent cut-off of a guided mode, to increase a reflectance of electrons entering energy barriers and to improve a temperature characteristic of a semiconductor laser.
In order to accomplish the aforementioned object, a semiconductor light-emitting device of the present invention includes an active layer for generating a light and a pair of cladding layers. The active layer has a multi quantum well structure in which a plurality of barrier layers and a plurality of quantum well layers are alternately arranged. The paired cladding layers sandwiches the active layer to confine the light within the active layer. Each of the cladding layers has a multi quantum barrier structure in which a plurality of barrier layers and a plurality of well layers are alternately arranged, and the multi quantum barrier layers of each of the cladding layers are graded or stepwise in configuration.
As the graded or stepwise MQB structure is provided for the cladding layers sandwiching the active layer, cut-off of a guided mode can be prevented, a reflectance of electrons entering the energy barriers can be increased and a temperature characteristic of a semiconductor light-emitting device can be improved.